Adjusting `\f`

[Werner LEMBERG]

I would like to change the definition of `\f` to something like this

```tex
"f" = #(make-music 'AbsoluteDynamicEvent
                   'text "f"
                   'tweaks '(((DynamicLineSpanner . horizon-padding) . 0.1)))

{ \stemDown f'4\f f'4\p }
```

to handle issue #4159.

However, if I do the above the `horizon-padding` property can no
longer be overridden with `\tweak` or `\override`.  How can I fix
that?  In other words, I would like to set the `horizon-padding`
property for `\f` as if I provided a default value for the
`DynamicLineSpanner` grob in `define-grobs.scm`.


    Werner